One of ordinary optical transmission systems is a wavelength division multiplexing (WDM) system. In the WDM system, an optical transmission device on a transmitter side multiplexes a plurality of optical signals with different respectively wavelengths and provides the multiplexed signal to one optical fiber. An optical transmission device on a receiver side separates the WDM signal received from one optical fiber into optical signals with the individual wavelengths, and a photo detector therein receives each of the separate optical signals with the respective wavelengths. An optical transmission device which relays a WDM signal may be provided between the devices of the transmitter side and the receiver side in some cases.
The device on the receiver side may amplify a WDM signal by means of an amplifier before separating the WDM signal into optical signals with respective wavelengths, and so may be the device for the relay. The optical transmission device described above monitors received optical power of the respective wavelengths included in the WDM signal before amplifying the WDM signal by means of the amplifier. The optical transmission device can have a grasp of working condition of the optical transmission device by calculating input power of optical signals with the respective wavelengths on the basis of a gain depending upon a working current of the amplifier. The amplifier is, e.g., a semiconductor optical amplifier (SOA).
A method for controlling an amplifier is to control a gain of the amplifier on the basis of a level of a dummy signal inputted to the amplifier, a level of the dummy signal amplified and outputted by the amplifier, and calibration data related to a gain characteristic of the amplifier. Another method is to separate a WDM signal amplified by the amplifier into a plurality of groups in accordance with the wavelengths, and to control the gain of the amplifier on the basis of input power calculated for each of the groups without using a dummy signal.
Related arts described above are disclosed in Japanese Laid-open Patent Application Publications No. 2008-166719 and No. 2001-144692.
The use of a dummy signal, however, is in need of a circuit which produces the dummy signal and a WDM optical signal filter which separates the dummy signal from other signals to be inputted to the amplifier, and thus the scale of the circuits is enlarged. Further, in order to separate a WDM signal having been amplified into a plurality of groups and to monitor received power for each of the groups, it is supposed to calculate the input power for each of the groups on the basis of the gain depending upon the working current of the amplifier. In such a case, the amplifier has characteristics of a change of the gain depending upon the input power and a gain variation depending upon the wavelengths, in such a way that the gain depends much upon the input power upon the working point coming close to a gain saturation area. Thus, a deviation of input power between wavelengths cannot be distinguished from a gain difference of the amplifier between the wavelengths, resulting in that the received optical power of the respective wavelengths included in the WDM signal can be monitored less precisely.